This invention relates in general to elements of cathode ray tubes and more particularly to indirectly heated cathodes for use in the electron gun structure of such tubes.
Cathode ray tubes conventionally employ one or more electron guns having indirectly heated thermionic cathode structures. Such cathodes usually contain a circular cap having a circular sidewall which fits around and engages an end portion of an elongated cylindrically shaped hollow sleeve along the interior of which is disposed an electrical heater element such as a twisted wire. The outer flat surface of the cap is coated with a suitable electron emissive substance. The heating element is energized in a well known manner by application thereof of a suitable d.c. voltage from a remote source, whereby the cap and emissive coating is heated to emit electrons. Typically, such tubes and sleeves are made of Nickel-Chromium alloy and are of thin wall construction. Examples of various prior art electron gun structures employing various arrangements of sleeves and caps are disclosed in U.S. Pat. No. 4,184,100 issued to Y. Takanashi, et al. on Jan. 15, 1980; U.S. Pat. No. 4,297,612 issued to Y. Anezaki on Oct. 27, 1981; and U.S. Pat. No. 3,549,929 issued to N. P. Pappadis on Dec. 22, 1970.
One difficulty that has been encountered using prior art sleeves and caps in indirectly heated cathode structures is that of low mechanical bonding strength between the cap and sleeve. We understand that many of these caps can be forced off the end of their corresponding sleeves by applying a push-off force to the cap while the sleeve is restrained of less than four pounds. Before these parts are molecularly bonded by firing. It would, of course, be highly desirable if the cap push-off strength could be substantially increased above such a level.
Another difficulty that has been encountered using such prior art sleeves and caps is the fact that a sharp or jagged edge on the end of the sleeve over and around which the cap is applied can cause damage to an end portion of the heating element by knicking or scrapping the same as, for example, when the cathode ray tube in which the gun is installed is subjected to vibration or mechanical shock.
By means of our invention, these and other difficulties encountered in the use of prior art sleeve and cap type indirectly heated cathodes are substantially reduced.